      double precision function dbesi1 (x)
c oct 1983 edition.  w. fullerton, c3, los alamos scientific lab.
      double precision x, bi1cs(17), xmax, xmin, xsml, y, d1mach,
     1  dcsevl, dbsi1e, dexp, dlog, dsqrt
      external d1mach, dbsi1e, dcsevl, dexp, dlog, dsqrt, initds
c
c series for bi1        on the interval  0.          to  9.00000e+00
c                                        with weighted error   1.44e-32
c                                         log weighted error  31.84
c                               significant figures required  31.45
c                                    decimal places required  32.46
c
      data bi1 cs(  1) / -.1971713261 0998597316 1385032181 49 d-2     /
      data bi1 cs(  2) / +.4073488766 7546480608 1553936520 14 d+0     /
      data bi1 cs(  3) / +.3483899429 9959455866 2450377837 87 d-1     /
      data bi1 cs(  4) / +.1545394556 3001236038 5984010584 89 d-2     /
      data bi1 cs(  5) / +.4188852109 8377784129 4588320041 20 d-4     /
      data bi1 cs(  6) / +.7649026764 8362114741 9597039660 69 d-6     /
      data bi1 cs(  7) / +.1004249392 4741178689 1798080372 38 d-7     /
      data bi1 cs(  8) / +.9932207791 9238106481 3712980548 63 d-10    /
      data bi1 cs(  9) / +.7663801791 8447637275 2001716813 49 d-12    /
      data bi1 cs( 10) / +.4741418923 8167394980 3880919481 60 d-14    /
      data bi1 cs( 11) / +.2404114404 0745181799 8631720320 00 d-16    /
      data bi1 cs( 12) / +.1017150500 7093713649 1211007999 99 d-18    /
      data bi1 cs( 13) / +.3645093565 7866949458 4917333333 33 d-21    /
      data bi1 cs( 14) / +.1120574950 2562039344 8106666666 66 d-23    /
      data bi1 cs( 15) / +.2987544193 4468088832 0000000000 00 d-26    /
      data bi1 cs( 16) / +.6973231093 9194709333 3333333333 33 d-29    /
      data bi1 cs( 17) / +.1436794822 0620800000 0000000000 00 d-31    /
c
      data nti1, xmin, xsml, xmax / 0, 3*0.d0 /
c
      if (nti1.ne.0) go to 10
      nti1 = initds (bi1cs, 17, 0.1*sngl(d1mach(3)))
      xmin = 2.0d0*d1mach(1)
      xsml = dsqrt (8.0d0*d1mach(3))
      xmax = dlog (d1mach(2))
c
 10   y = dabs(x)
      if (y.gt.3.0d0) go to 20
c
      dbesi1 = 0.0d0
      if (y.eq.0.0d0) return
c
      if (y.le.xmin) call seteru (
     1  38hdbesi1  dabs(x) so small i1 underflows, 38, 1, 0)
      if (y.gt.xmin) dbesi1 = 0.5d0*x
      if (y.gt.xsml) dbesi1 = x*(0.875d0 + dcsevl (y*y/4.5d0-1.d0,
     1  bi1cs, nti1))
      return
c
 20   if (y.gt.xmax) call seteru (
     1  35hdbesi1  dabs(x) so big i1 overflows, 35, 2, 2)
c
      dbesi1 = dexp(y) * dbsi1e(x)
c
      return
      end